Irrespective of the use of the drive device, the component which is at rest relative to the force introduction is denoted as the frame and the component which is pivotable thereto is denoted as the output.
In vehicle seats, it is known to use four arm kinematic systems as height adjusters, in which the rear upper joint is locked due to the moment load which is generally smallest at that point and for reasons of constructional space. Generally, this is carried out by locking modules which have little clearance or are free of clearance or load-bearing gear motors which are mounted on the frame and which act via a pinion on teeth on the rear rocker (=output), whereby ultimately free clearances or elastic clearances are again produced in the kinematic system. As the wobble geared fittings developed for adjusting the backrest are, in principle, free of clearance and are able to tolerate the required operating and crash loads in the upper rear point of articulation of conventional height adjuster-kinematic systems, it has already been proposed to produce a drive device in which such a geared fitting, as a load bearing joint which is free of clearance, is associated with the rear rocker and/or its upper point of articulation, the rocker itself or an intermediate lever being fixedly connected to the wobble fitting part and being used as a force transmitting element. One solution is disclosed in DE 102 25 247 A1, according to which a geared fitting may be incorporated in the kinematic system as a drive device by means of a lever and slot-pin guide.
One solution disclosed in DE 103 41 001 A1, which serves for reducing the kinematically produced non-uniformity due to the wobble motion of the geared fitting, is the use of an intermediate gear with a gear ratio which is non-uniform over one revolution, in order to increase the input speed of the geared fitting in phases with the kinematically produced low output speed of the drive device, although the actual drive (electric motor or step-by-step mechanism) operates at the same speed.
The immediate consequence of any construction with a direct connection of a wobble fitting part to a kinematic system by means of a lever is, however, a non-uniform speed of movement and direction of movement at the point of articulation of the lever, as the effective operating length of this lever as well as the point of force introduction are significantly altered, when observed over one revolution. During conventional backrest use, the lever (namely the backrest) typically with a length of 600 mm is markedly longer than the alteration to the lever length during rotation (the engagement radius of the teeth in the geared fitting of approximately ±30 mm corresponds to an alteration of 600 mm/±30 mm=±5%), in height adjuster-kinematics with typical lever lengths of approximately 100 mm the same amount of alteration of the lever length has a significant influence (100 mm/±30 mm=approximately ±30%). Even if, by careful choice of the kinematic points of articulation, this significant alteration to the force ratio and speed ratio may be reduced (approximately halved) in its effect, a 15% alteration to the speed ratio or force ratio within a kinematic system is disadvantageous as regards uniform adjustments.